Single core cable installation

[mrbj]

On low voltage systems, (UK 400V, 3ph+N ) and using Single core/Copper conductor/Aluminuim outer cables, 2per phase.
What is the standard practice for glanding off the Alum' armour ie..
a. Gland,terminate and bond both ends to earth, and use Al armour as earth return. or..
b. Gland, terminate and bond just one end to earth, the other end into an insulated gland, and use a seperate copper conductor for earth return.
I see no reason (since this is low voltage) to use option 'a' above, but, a colleague insists on option 'b'.
Your response would be appreciated.
Regards,
mrbj.

 

[waross]

Option "B". The current will induce voltages in the aluminum sheath. If there is a complete circuit then circulating currents will flow in the aluminum sheath. If these currents are allowed to flow they will add to the heating of the cable. In Canada, an installation of sheathed, single conductor cables must be derated if sheath currents are allowed to flow.
Normal practice in Canada is to insulate one end to prevent circulating currents and use the cable without derating.
Another issue you should be aware of if your load is such that two cables per phase are required. Induction heating of the metal switchgear at the feed end. There are two principle methods of avoiding this issue.
1. A hole is cut out of the side of the switchgear and an aluminum plate large enough to terminate all the cables is bolted in to place. As the aluminum is nonmagnetic it is not subject to the same magnitude of heating as the normal steel side or top of the switchgear.
2. Holes are punched in the steel side of the switchgear to install the cable glands. A hacksaw cut is then made between each group of "A" "B" "C" and "N" holes. This in effect makes one odd shaped hole for each group of cables to enter. The magnetic fields are able to cancel each other and heating is avoided.
yours

 

[waross]

This issue has been discussed on this site within the last several months. You may be able to find the previous thread with a search of this site.
yours